The present invention relates to a polymer dispersion-type liquid crystal optical device which utilizes a change in the phase between light transmission and light scattering (light absorption). More particularly, the present invention is concerned with a liquid crystal optical device which exhibits excellent display characteristics such as a high contrast, a low driving voltage and sharpness, and a method for producing the same.
In general, a liquid crystal display has hitherto found a wide range of applications in wristwatchs, desk top computers, personal computers, television sets, etc. as display medium for a letter and an image by virtue of its features such as low power consumption, light weight and small thickness. In general TN- and STN-liquid displays, a liquid crystal is sealed into a liquid crystal cell comprising two sheets of transparent conductive glass substrates having an oriented film applied thereto and a predetermined seal provided between the substrates, and the assembly is further sandwiched between polarizing plates.
The above-described liquid crystal displays, however, have problems including that (1) the angle of visibility is small due to the necessity of two polarizing plates and a back light having a high power consumption is necessary due to lack of brightness, (2) the dependency upon the cell thickness is so large that it is difficult to increase the area of the display, and (3) the production cost is high because sealing of a liquid crystal into the cell is difficult due to a complicated structure, which limits a reduction in weight, a reduction in thickness, an increase in area, a lowering in power consumption and a lowering in cost of the liquid crystal display.
A great expectation on a polymer dispersion-type liquid crystal display which utilizes a liquid crystal/polymer composite file comprising a polymer matrix and, dispersed therein, a liquid crystal was placed as a liquid crystal optical device for solving the above described problem, and research and development of the polymer dispersion-type liquid crystal display has become eager in the art.
Such a liquid crystal/polymer composite film is mainly produced by the following methods.
(1) A method wherein a porous polymer is impregnated with a liquid crystal.
(2) A method wherein an emulsion prepared by dispersing a liquid crystal in an aqueous solution of polyvinyl alcohol is cast and dried (see International Publication No. WO83/01016).
(3) A method wherein a solution of a liquid crystal and a polymer dissolved in a common solvent is cast and a phase separation of the liquid crystal and the polymer is conducted accompanying the removal of the solvent (see International Publication No. WO85/04262).
(4) A method wherein the monomer in a mixture of a liquid crystal with a monomer is polymerized to provide a phase separation structure of the liquid crystal and the polymer (see International Publication No. WO85/04262).
Among the above-described methods, the method (2) has advantages that the production of the composite film is simple, the structure and film thickness are easy to control and an increase in the area is possible, and this method has been already put to practical use for the production of a light-controllable glass etc.
As shown in FIG. 1, in the liquid crystal/polymer composite film produced by the method (2), the liquid crystal is dispersed as a micro droplet into a matrix resin. When no voltage is applied to the composite film, the liquid crystal molecules are arranged along a spherical wall of the matrix as shown in FIG. 1A and incident light is scattered at the inside and interface of the liquid crystal droplet due to a birefringence property of the liquid crystal molecule. This causes the liquid crystal/polymer composite film to become opaque.
The application of a voltage causes liquid crystal molecules to align in the direction of an electric field as shown in FIG. 1B, so that incident light travels straight to cause the liquid crystal/polymer composite film to become transparent.
The voltage necessary for aligning the liquid crystal molecules in the direction of an electric field depends upon the diameter of the liquid crystal droplet. Specifically, the constraining force which the liquid crystal droplet receives from an outer wall becomes relatively strong with a reduction in the size of the liquid crystal droplet, so that a higher electric field is necessary to align the liquid crystal in the direction of an electric field. For this reason, when a large liquid crystal droplet and a small liquid crystal droplet co-exist in the liquid crystal/polymer composite film, a curve showing a change in the parallel ray transmission with the voltage becomes gentle. On the other hand, when the diameter of the liquid crystal droplet is homogeneous, the application of a voltage gives rise to a sharp change in the parallel ray transmission.
The capability of the liquid crystal droplet to scatter incident light with no voltage being applied depends upon the diameter of the liquid crystal droplet. This ability is related to the relationship between the wavelength of light and the size of the liquid crystal droplet and the number of the interfaces of liquid crystal/resin. No satisfactory light scattering capability can be obtained when the size of the liquid crystal droplet is excessively large or small. Therefore, in order to simultaneously realize a high contrast, a low driving voltage and a high sharpness, it is necessary to conduct optimization through homogenization of the diameter of the liquid crystal droplet.
The diameter of the liquid crystal droplet in the liquid crystal/polymer composite film is determined by the diameter of the liquid crystal particle in the emulsion to be cast. Therefore, in the liquid crystal/polymer composite film as well, in order to control the diameter of the liquid crystal droplet, it is necessary to control the diameter of the liquid crystal particle in the emulsion.
According to a conventional method wherein an emulsion comprising a liquid crystal dispersed in an aqueous polyvinyl alcohol (PVA) solution is cast and dried, it is a matter of course that a phase separation structure comprising a matrix polymer of PVA and, dispersed therein, a liquid crystal particle is formed. Since, however, the transparency of PVA is so poor that a visual light transmission exceeding 70% cannot be attained (see Koh-ichi Nagano et al., "Poval", p. 378, Kobunshi Kankokai, 1981). When a cast film is prepared from an aqueous PVA solution, since it is very difficult to dry the film, the electrical insulation is low although it depends upon the production conditions, and it is generally said that the resistivity is about 10.sup.7 to 10.sup.9 .OMEGA..multidot.cm (see Tadahiro Miwa, "Gosei Jushi no Kagaku", p. 105, Gihodo, 1975). Further, with respect to the weatherability as well, the PVA is inferior to an acrylic polymer.
Thus, the liquid crystal/polymer composite film prepared according to the conventional method has problems of optical characteristics, electrical characteristics and durability.
The liquid crystal emulsion has hitherto been prepared by a mechanical emulsification method wherein use was made of a high-speed agitator and an emulsification method wherein use was made of an ultrasonic homogenizer. In these methods, however, it was difficult to narrow the particle diameter distribution of the dispersed liquid crystal. For this reason, the conventional liquid crystal/polymer composite film has a drawback that it needs a high driving voltage and is poor in sharpness. This drawback has bean an obstacle to a reduction in the cost of the display and the realization of multiplex driving.
Further examples of the problems with the conventional method for producing the liquid/polymer composite film include large limitation and difficulty in the production of the composite film.
In the production method (2), the liquid crystal dispersion is an aqueous thixotropic solution. For this reason, the coating property is generally so poor that it is not easy to form a homogeneous liquid crystal layer due to the inclusion of bubbles or the like. Further, it is difficult to increase the solid content of the liquid crystal dispersion, and in order to form a liquid crystal material layer having a thickness of several .mu.m to ten-odd .mu.m on a dry basis, it is necessary to form a layer having a thickness of ten-odd .mu.m to several tens of .mu.m, so that drying for a sufficiently long period of time is necessary.
Further, in the liquid crystal/polymer composite film produced by casting, since edge protrusions usually exist around the peripheral portion, the peripheral portion should be cut out in the production of a sandwich cell. For this reason, it is impossible to provide a liquid crystal/polymer composite film only in a necessary region on the substrate. Since the liquid crystal is expensive, this gives rise to a large problem of the production of a display cell and the production cost.
When a polymer dispersion-type liquid crystal display is used as a high-quality display panel, foreign matter, bubbles, unevenness of coating, etc. should not exist. However, the present situation is that the difficulty of defoaming, ccating/drying, etc. leads to large problems of productic rate, percentage non-defective, etc., so that a high-quality device cannot be produced at a low cost.